There are essentially two methods used to backlight transparent mode LCD panels, namely direct-lit (sometimes referred to as “array-lit”) and edge-lit. As is known a direct-lit LED backlight provides a surface of illuminating light behind the entire image-producing area of an LCD panel used in a television, computer monitor or similar display apparatus, whereas an edge-lit backlight provides lighting from the edge of the LCD panel.
Due to the small and point source nature of conventional LED bulbs and chips many individual LED bulbs or chips (“LED Units”) are needed to form the desired surface of light to adequately illuminate the relatively large image area involved. Typically, the LED units comprising the direct backlight are geometrically arranged in a simple checkerboard or skewed (“criss-cross”) grid pattern. Further, the LED units are electrically connected in series or “strings” in order to match the relatively low forward voltage drop of the individual LED units to the typically higher and available power supply voltage within the television or video monitor.
The white LED units (whether in bulb form or chip form) used in LCD backlights have a forward voltage drop characteristic of between approximately 3.0 and 4.0 volts. This characteristic is considered the “operating voltage” of the LED. All LEDs, whether white or color, are generally operated in constant current (“CC”) mode. This is because although LEDs have a fixed forward voltage characteristic, their light output is proportional to the current which flows through them, but not the voltage applied across them. This current is commonly referred to as the “operating current” of the LED. Unlike the more or less fixed operating voltage, the LED's operating current can be varied over a wide range in order to control the light output of the LED. In a typical direct LED backlight the operating current of each individual LED unit is in the range of tens of milliamps to hundreds of milliamps—depending primarily on the type of LEDs incorporated and the desired illumination level.
Most modern day electronic equipment incorporates an AC-to-DC power supply to derive the relatively low DC voltages needed by the equipment's internal electronic circuitry from the relatively high 60 Hz power mains of our public electrical distribution system. These “resident” DC power supplies typically produce an output in the range of 12 to 48 volts DC—considerably higher than the operating voltage (3-4 volts DC) of the white LEDs used in the subject backlights. Thus, a common design practice has developed of connecting the LED units in series-connected strings in which the forward voltages of the LEDs add arithmetically by well-known electric circuit laws, thereby achieving a voltage “match” between the LED operating voltage and the resident DC power supply. For example, if a power supply voltage of 24 VDC is present for other reasons in the equipment, the LED strings are composed of 6, 7 or 8 LED units to “match” the available supply voltage. Since a direct LCD backlight may need dozens or hundreds of LED units to fulfill the illumination area requirements, many such LED strings are utilized to fulfill that total required LED unit count. For example, a 20 inch diagonal LCD direct backlight might require 160 LED units to produce a sufficient and uniform backlight illumination surface. If the resident power supply is 24 VDC, the natural backlight topology would consist of 20 strings each containing 8 LEDs. Each string is equipped with its own CC driver circuit deriving its power from the resident power supply. All 20 strings with their associated CC drivers are essentially connected in parallel.
Such a typical backlight circuit topology is shown in FIG. 3. Six strings each containing eight LED units are shown. The LED symbols are shown in a physical rectilinear grid arrangement as is typical of contemporary industry practice. Current generated in the DC Power Supply is regulated to a fixed value by the constant current driver C, which connects to LED A, then LED B, and so forth, though to LED H and back into the negative side of the DC Power Supply. The LED units' operating current is thus set by the constant current source C, while their individual operating voltages are set by the E-I curve of the individual LED units.
This circuit topology, often referred to as “parallel-series strings”, is ubiquitous in the contemporary LCD backlight universe—both for edge-lit and direct types. Although the physical arrangement of the LED units in an edge-lit backlight is different than for a direct backlight, the same parallel-series string (“PSS”) circuit topology is found in edge-lit backlights for the same electrical reasons.
LED units can fail in two ways—open or shorted. The general cause of failures in backlights is overheating because of the physically constrained nature of the LED strings and the consequent lack of an adequate thermal transport mechanism to carry away the copious heat produced by the LED units. It is not known which failure mechanism is more dominant (failed-open or failed-short), but it is known that each type of failure has been experienced in real world equipment.
The effect of each failure type (i.e., open or shorted) is quite different when it occurs in a single LED unit in the ubiquitous string PSS topology. If a single LED unit fails open in a string, the complete string will extinguish. If a single LED unit fails shorted in a string, the remaining LED units will continue to illuminate at approximately their pre-failure output level. Obviously, the fail-open mechanism is the more onerous of the two, as a complete section of the dependent LCD image is lost to some degree to the viewer. The degree of loss is dependent on how much illumination is available from adjacent, still-operating LED strings. The detrimental impact of this partial loss-of-image on the end-use of the equipment depends on the criticality of the application and the type of image displayed. As an extreme example, in an air traffic control display, a dark (un-illuminated) bar-shaped area can hide aircraft momentarily or long-term depending on the direction of travel of the aircraft symbol across the screen.
Accordingly, a need exists for an LED illumination subassembly which overcomes the disadvantages of the prior art. The subject invention addresses that need by providing an LED illumination display which minimizes if not eliminates the effect of at loss of light from an LED string. For example, in accordance with one aspect of this invention the LED units are electrically connected in such a way that the loss of one string of LED units does not seriously diminish the picture content of the displayed image. In accordance with another aspect of this invention a “standby” LED unit automatically illuminates when a primary LED unit fails open. In accordance with yet another aspect of this invention, when a single LED unit in a string fails open, the remaining LED units of the string continue to operate normally.